The invention relates an arrangement for driving ultrasonic transducers in flow measuring apparatuses, wherein the ultrasonic transducer is acted upon by a sinusoidal wave.
Known from DE 100 48 959 C2 is such an arrangement in which the non-inverting input of an operational amplifier is driven by a sinusoidal signal, while the inverting input can be connected optionally through a changeover switch to one of two ultrasonic transducers.
With a transducer drive consisting of digital modules, however, the problem exists that the generation of a sinusoidal signal in a digital manner is very expensive. In the driving of a transducer with a square-wave signal, on the other hand, the harmonic wave components interfere, the latter at the same time exciting the transducer and thus being carried along by the transducer, so that an adulteration of the measurement signal arises. Beyond this, the driving of such a transducer with a signal containing a DC voltage component is undesired, as is the case with short bursts, since a resting signal typically has an average value at one of the operating voltage limits and the average value of the burst signal lies between the operating voltage limits, so that here a step function response is superimposed.
The invention is based on the task of creating an arrangement for the driving of ultrasonic transducers in flow measurement apparatus, which arrangement is not only cost effective but also allows a transducer operation with only negligible measurement errors.
Preferably, according to the invention, in a driving of ultrasonic transducers from a digital module, with the aid of a resonant circuit, preferably in the form of an LC ringing circuit, both a filtering of the driving sinusoidal signal as well as a CD-voltage suppression can be achieved.
Advantageously, with the invention there is, in particular, a filtering out of the harmonic waves from the drive signal, and furthermore the suppression of DC voltage components at the transducer through parallel connection of the ringing circuit coil. In addition, the efficiency of the transfer path is improved since the capacitance of the conduction cables to the transducer as well that of the transducer itself can be taken into consideration in the calculating of the LC member. As a result of the filter effect of the LC member, the noise ratio and thus the quality of the receiver signal can be substantially improved.
The present invention further relates to—and claims in a coordinate manner—an arrangement for signal processing in an ultrasonic flow meter according to the time-of-flight principle. The distinctive feature of this embodiment of the invention consists in the fact that a network is provided for the driving of an ultrasonic transducer arrangement. The transducer arrangement includes a first ultrasonic transducer and a second ultrasonic transducer. Furthermore, switching means are provided, whereby the first and second ultrasonic transducers are optionally switchable into either a transmitting operation or a receiving operation, and the network exhibits an electrical ringing circuit that is active as a frequency filter in the transmitting as well as the receiving operation of the respective ultrasonic transducer. In addition, the above-described embodiment ensures, in the receiving operation in the respective ultrasonic transducer, an amplification of the electrical signal generated by the ultrasonic transducer.
The present invention makes possible, first, the greatest possible suppression of the number of undesired harmonic wave components of the digital square-wave signals as drive signals coming from a digital module, and, through this, the avoidance of undesired sonic wave components in the water. Furthermore, ultrasonic transducers can be operated in either transmitting mode or receiving mode, whereby the direction discrimination for the ultrasonic signal takes place in dependence on the sequence in which the two ultrasonic transducers strings are shorted out. In this, in the transmitting operation of the transducer in question, the network serves to perform, through the resonant circuit of the network, a frequency selection or a bandwidth selection, and at the same time ensures an increase of the voltage present at the comparator in comparison to the voltage of the ultrasonic transducer with simple components.
Appropriately, for the embodiment of the resonant circuit a C-member is assigned to each ultrasonic transducer and, in addition, a common L-member is provided, so that the respective C-member and the L-member form a resonant circuit, according to the switch state. Such a circuit can likewise be realized through technologically simple switching means.
Appropriately, the switching means are assigned directly to each ultrasonic transducer.
An effective reduction of losses is assured in that the network is connected via a T-circuit to the signal conductor and to an input of the comparator, that the extension of the vertical branch of the T-circuit exhibits the L-member, and that the respective transversely-running part of the T-circuit exhibits the respective C-member, the switch, and the associated ultrasonic transducer. According to the switch position, the respective C-member thus forms a series or parallel circuit with the active ultrasonic transducer. The L-member supplements the capacitances to form a resonant circuit.
For optimization, i.e. for balancing of the above-described arrangement, the network displays in addition a further C-member, which is connected in parallel to the L-member. Through this means results a greater freedom in the dimensioning of the C-member in the T-circuit.
The network is appropriately arranged in such a way that it is in resonance with the fundamental wave of a drive signal of the network via the signal conductor, in order to achieve an optimum selection effect.
According to a further embodiment of the present invention, a symmetrical voltage for the driving of the comparator is applied to the input of the comparator, whereby the total amplitude of the input voltage in comparison to the input voltage at the network is increased or doubled, as the case may be.
Specifically, the arrangement described above displays a series circuit of an additional C-member and L-member. Here, the series circuit is wired parallel to the C-member and to the L-member (the extension of the vertical branch of the T-circuit). The second input of the comparator is driven via the series circuit. The series circuit, brings about, first, a voltage amplification through impedance transformation and, second, the possibility of the conversion of the sign of the voltage in the region of the input to the comparator.
Preferably, the network is a passive network with reciprocity in respect to the transmitting operation or the receiving operation of an ultrasonic transducer.
Finally, the present invention relates to an ultrasonic flow meter according to the time-of-flight principle, with a measurement section, means for transmitting and receiving ultrasonic signals, as well as means for further signal processing of the received ultrasonic signals.
The ultrasonic waves arriving via the measurement section at the transducer are converted into electrical voltage signals. Since the operating voltage of the transducer is too low for a further evaluation of the signal, the signal voltage is raised and fed to the further signal evaluation. For the voltage raising, conventional integrated semiconductor elements are used, which exhibit various disadvantages. For one thing, they possess nonlinearities, which can disadvantageously influence the measurement result, and for another thing, besides raising the voltage signal to be evaluated, they also raise the noise level. Beyond that, such semiconductor elements are comparatively expensive. In addition, semiconductor elements are always associated with greater current consumption.